Grazing Animals Needed to Protect Grassland Biodiversity

March 27, 2014 - News Release

Grasslands need grazers the way plants need light, says a new study by a global research team including a University of Guelph ecologist.

Protecting grasslands is particularly important as human activities threaten to over-fertilize these ecosystems and reduce their natural biodiversity, said Prof. Andrew MacDougall, Department of Integrative Biology.

He co-authored the study along with members of the Nutrient Network, a worldwide group of scientists working at 40 sites on six continents. The study was led by University of Minnesota researchers Elizabeth Borer and Eric Seabloom.

Their paper was published in Nature this week and appeared earlier online.

Burning of fossil fuels and crop fertilization has increased the amount of nitrogen in the atmosphere and in grasslands. From automobile exhaust to fertilizing of crops and lawns, he said, “Basically we’re fertilizing the planet.”

Over-fertilization can reduce diversity by allowing taller plants to dominate and shade out other species.

That problem might be offset by herbivores from sheep to zebras that crop taller plants, allowing light in for other species to grow. That helps maintain natural biodiversity, or a range of native plants as well as the plant-eaters and predators associated with them, said MacDougall.

He said humans are also altering natural predator-prey relationships. “We’re changing food webs everywhere. We’re losing major predators and replacing native grazers with domesticated ones such as sheep and cattle.

“In some regions of the world, grazing is being increasingly eliminated as farmers focus on crop production. What does this mean for grassland diversity?”

The researchers tested plots with and without added fertilizer, and with and without fences to control grazers. They measured the amount of plant material, the light reaching the ground and the number of species of plants.

In all study locations around the world – from the relatively pristine Serengeti to degraded grasslands in eastern North America -- plant diversity was higher where herbivores were able to graze normally.

He said the study helps ecologists understand the workings of seemingly simple grasslands, including the impacts of human disturbances. “We lack global understanding of what drives diversity in grasslands. They are simple in structure and rich in diversity. What factors drive that?”

The study might also aid conservationists and resource managers in recommending steps to preserve these ecosystems. “Grazing and grassland diversity go hand in hand. This study reinforces that idea.”

Grassland ecosystems generate about one-third of the Earth’s plant production and account for 70 per cent of farmland, underpinning global food production.